(1) Field of the Invention
The invention relates to liquid crystal display devices well suited for use in computer displays, television receivers and other industrial products and to methods for fabricating them. More particularly, the invention pertains to light-transmissive type and light-reflective type liquid crystal display devices capable of providing rapid response and a wide range of viewing angles and to fabrication methods thereof.
(2) Description of the Related Art
There have been practically used twisted-nematic (TN) liquid crystal display devices incorporating a nematic liquid crystal. The TN mode, however, has the drawback of poor response. Another disadvantage of the TN mode is that viewing angles, that is, angles through which the viewer can see images properly are narrow. Concretely, when diagonally viewing images in a TN liquid crystal display device, brightness and contrast decrease and gray scale inversion occurs. For this reason, such TN mode is unacceptable for liquid crystal display systems that operate at high speed to provide animatic images or require good angular viewability when viewed in diagonal directions. Another known type of liquid crystal display devices is the Polymer Dispersed Liquid Crystal (PDLC) mode that utilizes the effect of light dispersion. This mode advantageously provides high brightness, because it does not require use of a polarizing plate. However, the response speed of the PDLC mode is as low as that of TN liquid crystal display devices. Additionally, the PDLC mode provides a wide range of viewing angles but the viewing angles of the PDLC mode cannot be controlled in principle by a phase compensating layer like the TN mode. There have been developed other types of liquid crystal display devices: Ferroelectric Liquid Crystal (FLC) and Anti-Ferroelectric Liquid Crystal (ALFC). These modes suffer from the critical problems of poor shock resistance and temperature characteristics and therefore have not been put to practical use.
In an attempt to overcome the foregoing problems, Optically Compensated Bend (OCB) liquid crystal display devices have been proposed, which exhibit extremely rapid response and a relatively wide range of viewing angles. One example of such devices is disclosed in Japanese Patent Laid-Open Publication No. 7-84254 (1995). One embodiment of the OCB liquid crystal display devices according to this publication is designed as shown in FIG. 1 to have a liquid crystal cell 11 in which a liquid crystal 12 is enclosed between a pair of transparent substrates 13, 14 and in which a pixel electrode 15, a counter electrode 16 and alignment films 17, 18 are formed on the transparent substrates 13, 14. The surfaces of the alignment films 17, 18 are conditioned so as to form a bend alignment state in which liquid crystal molecules 12a, 12b proximate to or contacting the alignment films 17, 18 are symmetrically tilted as shown in FIG. 1. More concretely, the surfaces of the alignment films 17, 18 are rubbed in the same direction to form a pretilt angle ranging from several degrees to 10 degrees. The bend alignment state may include twist in the proximity of the centers of the transparent substrates 13, 14 (i.e., liquid crystal molecules in the proximity of the centers are twisted so that they do not lie in the plane where the X and Z axes lie) depending on design conditions. Provided on both sides of the liquid crystal cell 11 are polarizing plates 19, 20. Sandwiched between the transparent substrate 14 and the polarizing plate 20 is a phase compensating layer 21 for optically compensating the director alignment of the liquid crystal 12. In the above-described bend alignment state, the liquid crystal molecules change rapidly with a change in the driving voltage applied between the pixel electrode 15 and the counter electrode 16, and consequently, fast response can be achieved. Such fast response due to the rapid molecular change can be obtained even when changing applied voltage between two levels corresponding to two halftones that have a slight difference in brightness. The symmetry of the bend alignment state increases the angular viewability in the plane where the X and Z axes lie so that e.g., a viewing angle of about xc2x150xc2x0 can be achieved, whereas the phase compensating layer 21 increases angular viewability in the plane where the Y and Z axes lie so that e.g., a viewing angle of about xc2x140xc2x0 can be achieved. Note that, in FIG. 1, the X and Y axes designate the transverse direction and vertical direction, respectively, of the display screen. The phase compensating layer 21 also contributes to a reduction in driving voltage.
The OCB liquid crystal display device presents a difficult problem. That is, the device requires formation of the bend alignment state prior to image displaying, which is unfavorable for the following reason. When no voltage is applied between the pixel electrode 15 and the counter electrode 16, the bend alignment state is not formed but a splay alignment state P with the liquid crystal molecules arranged fanwise is created as shown in FIG. 2, even if the above surface treatment is applied to the alignment films 17, 18. Therefore, at the time such as when a power supply is turned on, the splay alignment state P should be changed to the bend alignment state Q by application of high electric energy. The transition from the splay alignment state P to the bend alignment state Q can be caused at relatively high speeds by applying a comparatively high voltage ranging from e.g., 10 V to 30 V between the pixel electrode 15 and the counter electrode 16. However, it takes more than tens of minutes to cause the transition when applying a voltage (several volts) that is low enough to avoid excessive load on the driving ICs. In the worst case, such transition does not occur until after an elapse of more than one hour. This delay hinders practical use of the OCB liquid crystal display device.
As an attempt to solve the above problem, Japanese Patent Laid-Open Publication No. 9-96790 (1997) proposes a technique in which the twisted alignment of the liquid crystal molecules as seen in the TN mode is combined with the rising alignment (in which the liquid crystal molecules are aligned in a direction normal to the substrates) similar to that of the OCB mode. This technique is intended to solve the above problem by eliminating the need for formation of the bend alignment state and to achieve higher response speed than the TN mode by forming a director alignment similar to the bend alignment state. In reality, however, fast response can not be necessarily achieved even if a director alignment similar to the bend alignment state is formed.
Although the above prior art OCB liquid crystal display device succeeds in providing wide viewing angles to a certain extent, the device still have difficulty in largely increasing the viewing angle within the plane where the Y and Z axes lie (see FIG. 1) by the phase compensating layer 21 alone and therefore the viewing angle characteristics vary significantly according to viewing directions. Accordingly, the OCB liquid crystal display device leaves much to be desired in the viewing angle uniformity. As mentioned earlier, the viewing angle within the plane where the X and Z axes lie (FIG. 1) can be improved by the symmetry of the bend alignment state. In order to further increase the viewing angles not only in this direction but also in other directions, it is conceivable to use a biaxial phase compensating layer as the phase compensating layer 21. However, the fabrication of such a phase compensating layer requires accurate control of the index of refraction in triaxial directions, so that where the OCB liquid crystal display device is applied to a large screen display system, it is extremely difficult to form such a compensating layer that possesses uniform properties throughout the display screen.
In many cases, the polarizing plates 19, 20 are placed as shown in FIG. 3 such that their polarization axes respectively form an angle of 45xc2x0 or a specified angle relative to the conditioning direction of the alignment films 17, 18. In this case, light incident on the liquid crystal cell 11 passes through the liquid crystal 12 in the birefringence mode. Such propagation tends to cause the viewing angle dependence of the hues of a display image (i.e., hues and color stability may vary according to viewing angles). Hue shifts would be caused not only by certain viewing angles but also by the following factors even when images are viewed squarely (i.e., in a direction perpendicular to the substrates). FIG. 4 shows the transmission rates of blue, green and red light where different voltages are applied between the pixel and counter electrodes of a liquid crystal display device. The liquid crystal display device used herein is produced under the following conditions:
Alignment film: Polyimide director alignment film PSI-A2204 produced by Chisso Corporation.
Liquid crystal: MT-5440 produced by Chisso Corporation.
Phase compensating layer: Biaxial oriented film produced by Nitto Denko Corporation.
Gap distance of a liquid crystal cell: about 5 xcexcm
Pretilt angle: 5xc2x0 to 6xc2x0
Other conditions:
(1) The upper and lower substrates are bonded such that the rubbing directions of the alignment films are parallel to each other.
(2) Wavelengths at the centers of the spectra of blue, green and red light are approximately 450 nm, 540 nm and 630 nm, respectively.
As shown in FIG. 4, the light transmission of the liquid crystal varies according to the wavelength of transmitted light. More concretely, when a voltage of 2 V is applied between the pixel and counter electrodes, the transmission rates of blue, green, red light are 0.08, 0.045 and 0.025, respectively. Accordingly, entire images on the screen become bluish. Although it is conceivable that hue shifts can be prevented by adjusting the voltage applied between the pixel and counter electrodes according to the color of light, such adjustment leads to an increased scale of the circuit and a higher production cost.
According to the first aspect of the invention, there is provided a liquid crystal display device comprising (1) a pixel electrode, (2) a counter electrode and (3) a liquid crystal enclosed between the pixel and counter electrodes,
wherein the respective opposed surfaces of the pixel and counter electrodes are conditioned such that liquid crystal molecules contacting or in the vicinity of the surfaces have specified pretilt angles,
wherein images are displayed by changing light transmission through formation of a bend alignment state of the liquid crystal, and
wherein a large pretilt angle domain is formed on at least either one of the surfaces of the pixel and counter electrodes, the large pretilt angle domain causing a larger pretilt angle of liquid crystal molecules than a region surrounding the large pretilt angle domain does.
One of objects of the invention is to quickly and reliably carry out the transition from the splay alignment state to the bend alignment state in a liquid crystal display device which displays images by changing light transmission through formation of the bend alignment state of the liquid crystal.
To accomplish this object, a liquid crystal display device according to the invention includes a large pretilt angle domain which is formed on at least either the surface of the pixel electrode or the surface of the counter electrode and which is conditioned such that the pretilt angle of liquid crystal molecules caused by the large pretilt angle domain is larger than the pretilt angle of molecules caused by the region surrounding the large pretilt angle domain. The liquid crystal molecules proximate to or contacting the large pretilt angle domain are comparatively raised, and therefore become a core for the transition from the splay alignment state to the bend alignment state when voltage is applied between the pixel electrode and the counter electrode. With this core, the transition region grows and expands, which enables the transition to occur reliably throughout the liquid crystal in a short time. In addition, such transition does not consume large amounts of electric energy so that the driver circuit is not subjected to excessive load.
To achieve the inventive effect, that is, the rapid, reliable transition, we tried to clarify the mechanism of the transition of the director alignment state. After a rigorous study, we found that just after application of voltage, the transition was more likely to occur in the vicinity of spacers which were disposed irregularly between the transparent substrates in order to keep a constant gap between the transparent substrates. The reason for this is that the alignment of the liquid crystal molecules proximate to the spacers tends to be irregular under the influence of the configuration of the spacers and other physical properties of their surfaces so that some molecules near the spacers have larger tilt angles than the tilt angle of the molecules far from the spacers. Such molecules trigger an occurrence of the transition from the splay alignment state to the bend alignment state in the neighborhood of the spacers. However, such transition is accidental and therefore does not occur in the neighborhood of every spacer. Moreover, the spacers may shift and are not necessarily positioned on all the pixels. Liquid crystal display devices usually have a multitude of pixels, and if parts of the pixels do not have such transition, sound images cannot be displayed. To solve this problem and achieve the high-speed, reliable transition, we have come to the idea of the provision of the large pretilt angle domain. Such a large pretilt angle domain may be formed, for example, by partially applying an alignment film material, which imparts a large pretilt angle to liquid crystal molecules, to the surface of an electrode, through phase separation or printing. Alternatively, it may be formed by providing a small projection on an electrode.
According to the second aspect of the invention, there is provided a liquid crystal display device comprising (1) a pixel electrode, (2) a counter electrode, (3) a liquid crystal enclosed between the pixel and counter electrodes, and (4) a phase compensating layer, wherein images are displayed by changing light transmission through formation of a bend alignment state of the liquid crystal and wherein the liquid crystal contains a chiral agent.
The above-described transition can be easily induced by adding a chiral agent to the liquid crystal. A combination of the large pretilt angle domain and the chiral additive causes the transition more easily.
According to the third aspect of the invention, there is provided a liquid crystal display device comprising (1) a first substrate having a pixel electrode formed thereon, (2) a second substrate having a counter electrode formed thereon and positioned opposite the first substrate, (3) a liquid crystal enclosed between the first and second substrates, (4) a first polarizer and a second polarizer disposed so as to sandwich the first and second substrates, the polarizing axes of the first and second polarizers crossing at right angles, and (5) a driver circuit for applying driving voltage between the pixel electrode and the counter electrode,
wherein the liquid crystal molecules of the liquid crystal have a twist angle ranging from 160xc2x0 to 200xc2x0, and
wherein the driver circuit applies driving voltage between the pixel and counter electrodes, the driving voltage being higher than the highest one of voltages that cause the maximal value of light transmission in the driving voltage-transmission characteristic of the liquid crystal display device.
According to the above liquid crystal display device of the invention, the twist angle of the molecules of the liquid crystal is in the range of from 160xc2x0 to 200xc2x0, and a voltage higher than the voltage that causes the extremum (maximal or minimal value) of light transmission in the driving voltage-transmission characteristic of the liquid crystal display device is applied between the pixel electrode and the counter electrode. With this arrangement, response as fast as that achieved by a device which forms the bend alignment state can be achieved without forming an alignment state similar to the bend alignment state. Concretely, since the liquid crystal molecules are kept in a twisted condition in the above device, there is no need to make a discrete phase transition such as the transition from the splay alignment state to the bend alignment state. Additionally, the liquid crystal molecules can be brought into an alignment state similar to the bend alignment state by application of the above-specified voltage. By virtue of this, images can be displayed, for instance, just after turning on the power supply of the liquid crystal display device and excellent response can be ensured.
According to the forth aspect of the invention, there is provided a liquid crystal display device comprising (1) a pixel electrode, (2) a counter electrode and (3) a liquid crystal enclosed between the pixel and counter electrodes,
wherein images are displayed by changing light transmission through formation of a bend alignment state of the liquid crystal, and
wherein pixels corresponding to the pixel electrode are divided into at least two domains which cause bend director fields having different orientations in the liquid crystal.
To improve viewing angle characteristics, thereby achieving good viewability in various directions in a liquid crystal display device which displays images by changing light transmission through formation of the bend alignment state of liquid crystal molecules, pixels corresponding to the pixel electrode are divided into at least two domains that cause bend director fields having different orientations in the liquid crystal. Such domain division can be accomplished by rubbing a plurality of regions on the alignment films in different directions or alternatively, by directing ultraviolet rays having different polarizing or illuminating directions onto the regions. With this arrangement, the self compensating ability of viewing angles inherent in the bend director alignment is exerted in a plurality of different directions so that good viewability in various directions can be ensured. Further, a phase compensator may be used in conjunction with the above arrangement to improve the viewing angle characteristics.
According to the fifth aspect of the invention, there is provided a liquid crystal display device comprising (1) a twisted liquid crystal cell having a liquid crystal layer sandwiched between a pair of substrates, the liquid crystal layer having liquid crystal molecules twisted between said pair of substrates and (2) a polarizing plate disposed on either the light incoming side or light outgoing side of the liquid crystal cell,
wherein said polarizing plate is disposed such that its polarizing axis is substantially parallel to the longitudinal axis of the liquid crystal molecules on the interface of one of said pair of substrates, said substrate being on the light incoming side or light outgoing side,
wherein the twist angle of the liquid crystal molecules in said liquid crystal layer is in the range of from 160xc2x0 to 200xc2x0 and said liquid crystal layer contains a dye or pigment,
which has a voltage-brightness characteristic according to which when the voltage applied to said liquid crystal cell exceeds the Freedericksz threshold voltage of the liquid crystal, brightness first rises gently with a first gradient and then rises with a second gradient sharper than the first gradient, and
which performs image displaying with applied voltages at least higher than the voltage corresponding to the turning point where brightness changes from the first gradient to the second gradient.
To solve the problems of (i) the difficulty in causing the transition from the splay alignment state to the bend alignment state, (ii) the difficulty in fabricating a phase compensator of excellent properties for improving the viewing angles and (iii) the viewing angle dependence of hues which results in hue variation and color instability according to viewing angles, the twist angle of the liquid crystal molecules should be in the range of from 160xc2x0 to 200xc2x0 or from 250xc2x0 to 290xc2x0, the liquid crystal layer should contain a dye or pigment, and image displaying is performed with driving voltage falling within a specified high range. According to the above arrangement, since the liquid crystal layer contains a dye or pigment, this liquid crystal display device utilizes the guest-host effect. Therefore, the above liquid crystal display device can overcome the viewing angle dependent of hues that is one of the outstanding problems imposed by the conventional OCB liquid crystal display devices incorporating the birefringence mode. In addition, the above display device is not in the birefringence mode, so there is no need to include a phase compensating layer. Use of the twisted liquid crystal cells enables the device to display images without the transition from the splay alignment state to the bend alignment state. Image displaying with driving voltage in a specified high range permits fast response and a satisfactorily high contrast.
According to the sixth aspect of the invention, there is provided a liquid crystal display device comprising (1) a plurality of pixel electrodes constituting a plurality of pixels, (2) a counter electrode, (3) a liquid crystal enclosed between the pixel electrodes and the counter electrode, and (4) a color filter having regions respectively corresponding to said pixels, each region transmitting any one of a plurality of colors,
wherein at least either the surfaces of the pixel electrodes or the surface of the counter electrode is conditioned such that liquid crystal molecules in the vicinity of the surfaces or surface are aligned so as to form specified pretilt angles,
wherein images are displayed by changing light transmission through formation of a bend alignment state of said liquid crystal, and
wherein said specified pretilt angles vary according to the colors of the pixels.
In the above liquid crystal display device, hue shifts caused by the dependence of the transmission of the liquid crystal on the wavelength of transmitted light can be overcome by setting pretilt angles according to the colors of the pixels. Specifically, different pretilt angles are formed for the three primary colors so that the same voltage-transmission characteristic can be attained for the three primary colors.